The present invention relates generally to push-pull amplifiers, and more particularly to a push-pull amplifier which operates with high efficiency, produces an output of low distortion factor, and carries out improved class "A" amplifier operation.
In general, amplifiers are classified as class "A", class "AB", class "B", etc., depending on the manner in which their operating points are selected. Of these amplifiers, a class "A" amplifier has an operating point which is set at substantially the middle of the linear part of the input voltage versus output current characteristic curve of the output-stage transistors. For this reason, a class "A" amplifier has the characteristic of a very low distortion but, on the other hand, has certain drawbacks such as the necessity of passing therethrough an idling current even when there is no signal, whereby the efficiency is poor, and a great quantity of heat is generated. In the case of an ordinary class "A" amplifier of 100 W output, for example, the power-source voltage is set at .+-.50 V, and the idling current is set at a value of the order of 4 A. Consequently, at the time when there is no signal, a power of 400 W is consumed. For this reason, a great quantity of heat is generated in a class "A" amplifier, which necessitates elaborate means for dissipating the heat, giving rise to problems such as high cost, increased weight, and difficulty in miniaturization.
Furthermore, the idling current of a class "A" amplifier is determined with consideration given to the estimated minimum value of the load impedance (for example, 4.OMEGA.). For this reason, in the case where the amplifier is used with a load impedance higher than this, there has been a disadvantage in efficiency as described hereinafter. Still another problem is that, when an output exceeding the rated value is produced by a conventional class "A" amplifier, its operation becomes one of class "AB", and switching distortion arises.
In the case of a class "AB" amplifier, the idling current is less than that of a class "A" amplifier, while in the case of a class "B" amplifier, the idling current is theoretically zero. In both of these amplifiers, the efficiency is higher than that in a class "A" amplifier, and less heat is generated. On the other hand, however, the output-stage transistors of the class "AB" and class "B" push-pull amplifiers are caused to undergo switching operation. For this reason, switching distortion is produced in their outputs, and the distortion factor is worse than that in a class "A" push-pull amplifier.